The present invention relates to optical technology, and more particularly to a method and system for providing an in-line optical circulator.
Conventional optical circulators are used for many purposes. For example, conventional optical circulators may be employed in systems transmitting optical signals in order to transmit optical signals in a particular direction. In a three port optical circulator, an optical signal input at the first port will be transmitted to the second port. An optical signal input at the second port will be transmitted to the third port. However, optical signals will not be transmitted in the reverse direction. For example, an optical signal input at the second port will not be transmitted to the first port. Optical circulators can also come in a variety of configurations. One desirable configuration is an in-line optical circulator in which the first and third ports are adjacent, while the second port is at the opposing side of the system.
One prior art optical circulator is described in U.S. Pat. No. 5,909,310 by Li, et al and shown in FIG. 1A. This conventional optical in-line circulator 10 includes a first port 12, a second port 14 and a third port 16. The conventional optical in-line circulator 10 also includes a first collimator 18, a first birefringent crystal 20, a first pair of half wave plates 22A and 22B, a first Faraday rotator 24, a conventional Wollaston prism 26, a second birefringent crystal 28, a second Faraday rotator 30, a second pair of half wave plates 32A and 32B, a third birefringent crystal 34, a second collimator 36 and the fiber for the second port 14.
This conventional optical in-line circulator suffers from disadvantages. First, the optical axes half wave plates 22A and 22B in the first pair of wave plates and the first birefringent crystal 20 need to be aligned to each other. Similarly, the optical axes of the half wave plates 32A and 32B in the second pair of wave plates and the second birefringent crystal 34 also need to be aligned to each other. These alignment procedures that are required in the manufacturing process are complicated and difficult. Therefore, the tolerance of the relative orientation of the directions of the optical axes of the wave plates 22A, 22B and 32A, 32B are relatively high, which yields a lower isolation. Thus, manufacturing is made more complex and expensive. In addition, temperature dependent phase retardation for the half wave plates 22A, 22B, 32A and 32B gives the circulator a narrower temperature bandwidth for isolation.
U.S. Pat. No. 6,049,426 by Xie et al. (xe2x80x9cXiexe2x80x9d) describes another conventional in-line optical circulator. FIG. 2 depicts a conventional in-line optical circulator 50 in accordance with the teachings of Xie. It does not utilize any half wave plates and eliminates one birefringent crystal. However, the optical circulator of Xie uses an additional Wollaston prism 52 having wedges 52A and 52B. One of ordinary skill in the art will readily realize that the conventional in-line optical circulator 50 is relatively difficult to manufacture with higher cost. The optical circulator 50 suffers from two drawbacks. First, the optical circulator 50 uses two Wollaston prisms 26xe2x80x2 and 52. As described above, Wollaston prisms 26xe2x80x2 and 52 are relatively difficult and expensive to manufacture. The cost is thus increased by the additional number of Wollaston prism. Thus, although the half-wave plates 22A, 22B, 32A and 32B have been eliminated, the addition of a Wollaston prism still renders the optical circulator of Xie expensive and difficult to manufacture. Second, since the beam deflection angular tolerance introduced by Wollaston prisms is accumulated with the number of Wollaston prisms used, the beam deflection angular tolerance introduced by Wollaston prisms 26xe2x80x2 and 52 in circulator 50 is doubled compared with the circulator with only one Wollaston prism, making optical alignment and, therefore, manufacture more difficult and complex.
Accordingly, what is needed is a system and method for providing an optical circulator that is simpler to manufacture with a lower cost. The present invention addresses such a need.
The present invention provides a method and system for providing an optical circulator. The optical circulator comprises a first port, a second port and a third port adjacent to the first port. The optical circulator also comprises a first birefringent material, a first rotator pair, a polarization beam deflector, a second birefringent material, a second rotator pair and a third birefringent material. The first birefringent material is adjacent to the first and third ports. The first rotator pair, second birefringent material, second rotator pair and third birefringent material follow in order, with the third birefringent material being closest to the second port. The first birefringent material is optically coupled to the first port and the third port and has a longitudinal axis, a transverse direction perpendicular to the longitudinal axis, a first displacement direction and a first length. The first displacement direction is at a first oblique angle from the transverse direction. The polarization beam deflector changes the direction of the optical signal without introducing a walk-off in the optical signal. The second birefringent material having the longitudinal axis and a second displacement direction, the second displacement direction being perpendicular to the longitudinal axis. The third birefringent material has the longitudinal axis, the transverse direction perpendicular to the longitudinal axis, a third displacement direction and a second length. The third displacement direction is at a second oblique angle from the transverse direction. As a result, a first optical path is established from the first port to the second port, and a second optical path is established from the second port to the third port such that when an optical signal is input at the first port the optical signal travels along the first optical path to the second port and when the optical signal is input to the second port the optical signal travels along the second optical path to the third port.
According to the system and method disclosed herein, the present invention provides an in-line optical circulator which can be more easily and cheaply manufactured than conventional in-line optical circulators. In particular, the optical circulator can be made with only a single polarization beam deflector and without the use of any half-wave plates, making the optical circulator in accordance with the present invention more economical, simpler to fabricate and have better performance for isolation.